In general, violet-blue to blue colors used for dyeing or hueing agents for materials such as plastics, fabrics, and hair are either fused polycyclics such as triarylmethanes, substituted phenoxazines and anthraquinones, carbocyclic azo dyes in which the acceptor half of the molecule is highly substituted with electron-withdrawing groups used to shift the color into the desired range, bis(azo) compounds that extend conjugation, or azo dyes that have a carbocyclic donor portion and a thiazolium or thiazole (substituted with electron-withdrawing groups such as cyano or nitro) acceptor portion. Considering the donor-acceptor approach of color design, the well-known tactics are to include powerful election-withdrawing groups in the acceptor end of the molecule, thus pulling electron density towards the acceptor portion, driving the color to higher absorbance wavelengths. Although it can give the desired color shift, it can decrease the stability of the dyes, particularly when the acceptor end is a heterocycle such as imidazolium or thiazolium.
Although these well-known and well-accepted aforementioned classes of dyes provide the violet-blue to blue colors desired, each of these classes has at least one flaw that makes it inappropriate for dyeing at alkaline pH in highly aqueous systems, such as in hair dye products or highly aqueous dyebaths. (More aqueous dyebaths are desired to decrease the environmental impact relative to dyebaths that use high levels of organic solvents.) The solubility, fastness properties, or stability of current materials may be insufficient for the applications. In cases in which solubility, fastness, and stability are sufficient, the current materials do not have the required blue hue. Specifically in applications such as hair dyeing, this is an important outage.
Thiazolium azo and thiazole azo compounds in particular provide brilliant colors with high intensity. However, it is well-known that particularly for the vibrant and water soluble thiazolium dyes, stability is poor. Common, commercial imidazolium dyes (analogs of the thiazolium dyes) can give brilliant and stable orange to violet-red colors. However, there are no good examples of blue imidazolium dyes, most likely because using the common approach of adding electron-withdrawing groups to the acceptor (imidazolium) end of the molecule results in materials that are highly unstable, especially at alkaline pH.
Baumann and Dehnert described generating a violet imidazolium azo dye (in Chimia 15, 1961, 163-168) however they could not obtain blues, and Abbot, et al. (in J. Phys. Chem. J. Phys. Chem. A 2013, 117, 1853-1871) demonstrated the inherent instability of thiazolium compounds under basic conditions, making them unsuitable for applications for which neutral to alkaline pH is necessary.